The news follows weeks of rumors and speculation about the fates of the late-night hosts.

"Congratulations Jimmy," Leno said in a statement. "I hope you're as lucky as me and hold on to the job until you're the old guy. If you need me, I'll be at the garage."

Fallon added: "I'm really excited to host a show that starts today instead of tomorrow."

"Jay Leno is an entertainment icon, making millions of people laugh every weeknight for more than 20 years," said Steve Burke, chief executive officer of NBCUniversal. "His long reign as the highest-rated late-night host is a testament to his work ethic and dedication to his viewers and to NBC.

"We are purposefully making this change when Jay is No. 1, just as Jay replaced Johnny Carson when he was No. 1. Jimmy Fallon is a unique talent and this is his time. I'm thrilled he will become the sixth host of 'The Tonight Show' at exactly the right moment, in conjunction with our coverage of next year's Winter Olympic Games from Sochi, Russia."

Fallon debuted as host of "Late Night with Jimmy Fallon" in March 2009. NBC did not say who would replace him in the 12:35 a.m. time slot.